FIBEROPTIC COUPLING

Abstract: Fused fiberoptic couplers of the type formed by the processes of providing the fibers, assembling them in a close-packed relationship, and heating and drawing the assembly of fibers in the close-packed region. The fiberoptic couplers (37) are fabricated from optical fibers that have optical cores (38, 46, 48, 50) and surrounding cladding and/or spacer fibers that have no core (40, 42, 44). The number of optical and spacer fibers and their relative arrangements are selected to achieve desired properties of the coupler (37). Its has been discovered that the steps involved in forming such a coupler, introduce slight size variation that can seriously affect the performance of the coupler, and that these slight variations can be accommodated by slight, controlled, under-sizing of one or more of the surrounding fibers or over-sizing of the central fiber. Other fiberoptic couplers, which may or may not be formed in the above manner, are described.

Description: FIBEROPTIC COUPLING Background The invention relates to fiberoptic coupling. Fiberoptic couplers have been developed for the purpose of splitting the optical power available on a single fiber into several outputs. In a fiberoptic coupler, optical power in an input optical fiber becomes distributed among all the optical fibers in the structure. Light not coupled to the neighboring fibers remains in the input fiber. The term "coupling ratio", as used herein, is defined as the ratio of the optical power in any one output fiber to the total output optical power in the coupler. Fiberoptic couplers are fabricated by fusing two or more optical fibers together in a coupling region. Successful coupler fabrication depends upon controlling the placement of the fibers brought into contact with each other in the coupling region prior to fusing. Imperfections such as dirt on the fiber surfaces, imbalances in tension forces applied to the fibers prior to the application of fusion heat, and three-dimensional non-uniformities in the applied fusion heat can reduce the coupler fabrication yield. Fused couplers have been made from seven cylindrical fibers, in which six fibers surround a seventh central fiber. Due to the circular cross-section of the fibers, six identical fibers can be arranged around an identical central fiber so that each of the surrounding fibers will contact two neighboring surrounding fibers and the central fiber. The term "cross-section", as used herein, is defined as a cut lying in a plane perpendicular to the longitudinal axis of the coupler (i.e., the direction of propagation of light in the coupler) . Mutual contact among the fibers promotes uniform fusion, and results in greater output power uniformity. The term "output power uniformity", as used herein, refers to the output power difference between the output fiber with the maximum optical power and the output fiber with the minimum optical power. As taught by Stowe et al. (1992, U.S. Patent No. 5,121,452, by one of the inventors here) unitary couplers of the form 1XN, with N fibers surrounding a central fiber, may be fabricated if the diameter d of the surrounding fibers is related to the diameter of the central fiber D according to the following diameter- ratio-equation: d/D = sin(rτ/N)/(l-sin(τr/N)) . For example, couplers employed in distribution systems that require splitting in multiples of four (e.g., 1X4, 1X8, 1X12, and 1X16 port configurations) may be fabricated by sizing fibers according to the above equation. Summary In one general aspect, the invention concerns fused fiberoptic couplers of the type formed from axially elongated, cylindrical fibers of transparent substance each having a circular transverse cross-section of preselected diameter. The fibers include a central fiber and at least one ring of N surrounding fibers. At least some of the fibers are optical fibers that have a core and a surrounding cladding. The coupler is formed by the processes of providing the fibers, assembling them into a structure in which the constituent fibers have a close- packed relationship to one another, and then heating and drawing the assembly of fibers. We have discovered that the steps involved in forming such a coupler, introduce slight size variations that can seriously affect the performance of the coupler. and that these slight variations can be accommodated by slight, controlled, under-sizing of one or more of the surrounding fibers or over-sizing of the central fiber. According to this aspect, the invention features a coupler characterized in that the sum of the diameters of the surrounding fibers is slightly less than the sum of diameters of a ring of N circles closely-packed about the central fiber in a plane transverse to the longitudinal axis of the central fiber in a manner that provides, during manufacture, slight space in the ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture a close-packed relationship of the fibers to one another is obtained. As used herein, the term "close-packed relationship" means that each surrounding fiber contacts adjacent fibers. By slightly reducing the diameter of the surrounding fibers beyond that specified by the above diameter-ratio-equation, the optical performance of fiberoptic couplers greatly improves. An output power uniformity of about 1^ dB may be achieved with a probability of about 90%. Improved output power uniformity relaxes the dynamic range constraints placed on the associated electronic circuitry, generally improves the performance of the overall optical system, and further allows the optical couplers to be employed in applications requiring better output power uniformity. Certain embodiments according to this aspect of the invention include one or more of the following features. A fusing process employed in the manufacture of the coupler preferably uses helical twist of the surrounding fibers to obtain stable contact between all neighboring fibers prior to fusion so that during the fusing process the neighboring surrounding fibers fuse together and to the central fiber in a fused region into a unitary optical structure. The amount by which the sum of diameters of the surrounding fibers is preferably less than the sum of the diameters of the ring of N circles is determined, at least in major part, to accommodate the slight elliptical shape of the cross-section of the surrounding fibers, taken transversely to the axis of the central fiber, attributable to their helical twist relative to the central fiber, so that the desired close- packing relationship of the fibers to one another is obtained. The helical twist preferably has a pitch in the range of about 0.5 cm to 5 cm and the amount by which the sum of diameters of the surrounding fibers is less than the sum of the diameters of the ring of N circles is substantially inversely proportional to the square of said pitch. The term "pitch", as used herein, is defined as the distance, measured along the longitudinal axis of the coupler, traversed by a surrounding fiber to complete one helical rotation about the central fiber. Each of the N circles in the close-packed ring of circles surrounding the central fiber preferably has a diameter d, the central fiber preferably has a diameter D, and the close-packing of the N circles preferably conforms substantially to a ratio of the diameters equal to the value d/D = sin(r/N)/(l-sin(τr/N) ) . After manufacture, the surrounding fibers may have substantially the same diameter, or at least two of the surrounding fibers may have different diameters. The processes of heating and fusing are preferably performed in a fused region, and the central fiber and the surrounding fibers are preferably formed of identical fibers that extend beyond the fused region. The difference in diameters of the fibers in the fused region is preferably the result of a substantially uniform decrease in the diameter of the surrounding fibers or a substantially uniform increase in the diameter of the central fiber, prior to fusion. At least one of the surrounding fibers may have a smaller diameter than the diameter of the central fiber. The reduction in the surrounding fiber diameter is preferably achieved by etching, drawing, preselection of a smaller diameter fiber (within the manufacturing tolerances) or a combination of these techniques. The increase in the central fiber diameter is achieved, prior to fusion, by the addition of a material to the outer surface of the central fiber that has a refractive index that is greater than or about equal to the refractive index of the cladding surrounding the central fiber. The surrounding fibers may be preselected to have a smaller diameter than the central fiber, or the central fiber may be preselected to have a larger diameter than the surrounding fibers. The coupler has a larger bandwidth of optical wavelength response and an improved output power uniformity relative to a coupler formed of the same fibers without the accommodation of the process-related shifts. In certain other embodiments according to this aspect, the process-related shift is based at least in part on an effective reduction in the diameter of the central fiber caused by inward indenting pressure of the surrounding fibers upon the central fiber during the heating and fusing step of manufacture. The central fiber may be constructed to serve as an input port for optical power, the surrounding fibers may each be constructed to function as an output port, and the formation of the fused region is controlled in order to achieve a desired coupling ratio. The desired coupling ratio is preferably characterized in that the fraction of output power that couples to each of the surrounding fibers is substantially 1/N or 1/(N+l) . The term "spacer fiber", as used herein, is defined as a fiber of transparent material that has a refractive index substantially matched to the refractive index of the cladding of the optical fibers, but has no optical core. Embodiments of this aspect of the invention preferably include a fiberoptic coupler comprising a multiplicity of optical fibers each having a core and a surrounding cladding, and a multiplicity of spacer fibers that each have a refractive index substantially matched to the refractive index of the cladding of the optical fibers. For example, at least the central fiber is a spacer fiber and at least some of the surrounding fibers are optical fibers, or at least the central fiber is an optical fiber and at least some of the surrounding fibers are spacer fibers. Certain preferred embodiments according to this general aspect of the invention include 1XM (M taking a value of 2, 3, 4, 5 or 6) and 1XN (N taking an integer value greater than 6) fiberoptic couplers characterized in that the sum of the diameters of the surrounding fibers is slightly less than the sum of diameters of a ring of N circles closely-packed about the central fiber in a plane transverse to the longitudinal axis of the central fiber in a manner that provides, during manufacture, slight space in the ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture a close-packed relationship of the fibers to one another is obtained. In a preferred coupler that includes two layers of surrounding fibers, one of the fibers (either an optical fiber or a spacer fiber) is surrounded by an inner ring of six optical and spacer fibers, and an outer ring of twelve optical and spacer fibers that are located substantially symmetrically around the central fiber. In the following aspects of the invention, the constituent fibers of a fiberoptic coupler may or may not be sized in the manner described above. In one general aspect, the invention features a fiberoptic coupler comprising a central spacer fiber that has no optical core, surrounded by a close-packed ring of multiple surrounding fibers at least two of which are optical fibers that have optical cores and surrounding cladding. All of the surrounding fibers and the central spacer fiber are fused together in their respective regions of contact in a limited length coupling region to form a unitary structure adapted to couple propagating modes of light between the optical fibers. In a particularly important aspect, the invention features a 1X4 fiberoptic coupler that includes a central optical fiber surrounded by a close-packed ring of fibers arranged in the following fiber sequence: a group of two adjacent optical fibers, a spacer fiber, a second group of two adjacent optical fibers, and a second spacer fiber; and in which the constituent fibers are fused together in their respective regions of contact in a limited length coupling region, thereby forming a unitary structure. Applicants have discovered that a 1X4 coupler can be fabricated in a simple and cost-effective manner. Applicants realized that a symmetrical arrangement of four optical fibers around a central optical fiber, which requires careful special sizing of the diameters of the constituent fibers, is not necessary to achieve a desired 1X4 coupling ratio between the optical fibers. Instead, Applicant's 1X4 structure can be manufactured with seven standard-size optical and spacer fibers. Thus, using Applicant's inventive approach, 1X4 couplers can be fabricated with high efficiency and reliability, and at low cost. According to another general aspect, the invention features a fiberoptic coupler assembly in which the optical fibers of the fiberoptic coupler are arranged into substantially optically segregated groups of two or more adjacent optical fibers, with spacer fibers serving to substantially eliminate the coupling of light between the different groups. According to yet another general aspect, the invention features a fiberoptic coupler comprising a central fiber surrounded by two close-packed rings of fibers selected from optical and spacer fibers. In another general aspect, a fiberoptic coupler comprising: an inner axially elongated spacer fiber having no optical core; and first and second axially elongated optical fibers each having an optical core and a surrounding cladding, the optical and spacer fibers being arranged linearly side-by-side with the spacer fiber separating and contacting the first and second optical fibers in a limited length coupling region, the coupling region comprising a fused region in which the optical and spacer fibers, in their respective regions of contact, are fused together forming a unitary ribbon-like structure, wherein the transverse distance separating the optical cores of the optical fibers is selected to achieve a desired wavelength-dependent coupling ratio between the constituent optical fibers of the coupler. Other features and advantages will be apparent from the following description and from the claims. Description Fig. 1 is a side view in, partial cutaway, of a 1X8 fiberoptic coupler formed of eight optical fibers of a substantially identical diameter arranged in a close- packed configuration about a central fiber in accordance with the present invention. Fig. 2 is a cross-sectional view of the fiberoptic coupler of Fig. 1 taken along line 2-2. Fig. 3 is a side view of the fiberoptic coupler of Fig. 1 formed by twisting, fusing, and drawing the constituent surrounding fibers. Fig. 4 is a cross-sectional view of the fiberoptic coupler of Fig. 3 taken along line 4-4. Fig. 5 is an illustrative, cross-sectional view of a seven fiber coupler, prior to fusion, in which the increase of the effective cross-sectional diameters of the surrounding fibers, due to helical twist of the fibers, has caused a shift in the geometric relationship of the constituent fibers to one another so that close- packing of the fibers is not possible. Fig. 5A is a cross-sectional view of a fiberoptic coupler in which the diameter of one of the surrounding fibers has been reduced to permit close-packing of the constituent fibers. Fig. 5B is a cross-sectional view of a fiberoptic coupler in which the diameter of each of the surrounding fibers has been reduced by different amounts to permit close-packing of the constituent fibers. Fig. 5C is a cross-sectional view of a preferred seven fiber coupler, prior to twisting of the surrounding fibers, in which the diameters of each of the surrounding fibers have been reduced by substantially the same amount to compensate for a subsequent helical twist of the surrounding fibers. Fig. 5D is a cross-sectional view of coupler of Fig. 5C, after the surrounding fibers have been helically twisted about the central fiber, thereby taking up the slight space in the ring of surrounding fibers in the coupler of Fig. 5C. Fig. 6 is a table that provides the reduction Δd in the diameter of each of the surrounding fibers for a given twist pitch P that is required to compensate for the helical twisting of the fibers during the assembly of 1X6 and 1X8 fiberoptic couplers. Fig. 7 is a cross-sectional view of a 1X3 fiberoptic coupler formed from optical fibers and spacer fibers arranged in a close-packed configuration in accordance with the present invention. Figs. 7A through 7G are cross-sectional views of various 1 x N or M x N embodiments of fiberoptic couplers which are formed from a central fiber surrounded by a single ring of fibers. Fig. 8 is a cross-sectional view of a 1X4 fiberoptic coupler formed from a surrounding ring of twelve close-packed spacer fibers. Fig. 9 is a cross-sectional view of a fiberoptic coupler formed from nineteen optical and spacer fibers arranged in a close-packed configuration in accordance with the present invention. Fig. 10 is a cross-sectional view of an 8X8 fiberoptic coupler. Figs. 10A through 10D are cross-sectional views of various embodiments of 1 x N or M x N fiberoptic couplers which are formed from a central fiber surrounded by two close-packed rings of surrounding fibers. Fig. 11 is a cross-sectional view of a seven fiber coupler in which the central fiber is surrounded by a sleeve to permit close-packing of the constituent fibers. Figs. 12 through 12A are cross-sectional views of several fiberoptic couplers separated by one or more spacer fibers, all integrated into one fiberoptic coupler structure. Figs. 13 through 13B are cross-sectional views of linear fiberoptic couplers in accordance with the present invention. Referring to Figs. 1 and 2, a fiberoptic coupler 10 is formed from a ring of eight cylindrical optical fibers 12, that have a substantially equal diameter, surrounding a central cylindrical optical fiber 14. Prior to fabrication, the central and surrounding optical fibers begin as single-mode telecommunications fibers of about 125 μm outer diameter, that include cores 16 and 18, and cladding 20 and 22, respectively. The selected optical fibers may be single-mode or multi-mode optical fibers. The diameter of the surrounding fibers, and the diameter of the central fiber D, are pre-sized (e.g., by etching, by drawing as described by Stowe et al., in U.S. Patent No. 5,121,452 (assigned to the present assignee) which is incorporated herein by reference, by employing a combination of etching and drawing, by controlling the twist rate, by deposition, by longitudinal compression, or by adding a sleeve of e.g., cladding material, of appropriate thickness about the central or surrounding fibers) . The sum of the diameters of the surrounding fibers is slightly less than the sum of diameters of a ring of a number of circles, equal to the number of surrounding fibers, that each have a diameter d and that are closely- packed about the central fiber in a plane transverse to the longitudinal axis of the central fiber. The ratio d/D, of the diameter of the circles to the central fiber diameter, for coupler 10 is about 0.62, as given by the diameter-ratio-equation: d/D = sin(τr/8)/(l-sin(π/8)) . (1) The sum of the diameters of the surrounding fibers 12 are reduced by a small amount (e.g., by the above- mentioned techniques) relative to the sum of the diameters of the above circles in a manner that provides, during manufacture, slight space in the ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, as explained in detail below, so that, after manufacture, a close-packed relationship of the fibers to one another is obtained. During fabrication, coupler 10 is fused and drawn to form a tapered region 23 and a coupling region 24 (Fig. 3) , in which the original ratio of diameters d/D is essentially preserved (Fig. 4). Prior to the fusing process, the individual pre-sized fibers are arranged and aligned in a close-packed configuration as shown in Figs. l and 2. The coupler is mounted into the clamps of two translation stages. A heat source (e.g., a torch, an electrically heated resistive wire, an electric arc, or a laser) is applied over a relatively narrow region of the coupler until a suitable working temperature is obtained, at which point one or both translation stages are moved apart to elongate the heated region (these processes are normally referred to as unidirectional or bi-directional drawing, respectively) . During the fuse-drawing process, light is coupled into one of the optical fibers at the input side of the coupler, and is detected in at least one optical fiber at the output side of the coupler. The drawing is continued until the amount of light coupled between the input and output fibers reaches a desired relationship. To create a radial force that facilitates uniform fusing of the constituent fibers the surrounding fibers may be helically twisted as shown in Fig. 3, prior to, or during fusion. Depending on the type of coupler, or the desired coupling ratio, the coupling region may have a fractional turn or several turns. The pitch of the surrounding fibers is typically between 0.5 cm and 5 cm. During the fuse-drawing process, the diameter of the coupling region can be reduced by more than 75%. The fused portion of the resulting coupler has a length of several millimeters to a few centimeters, depending in part upon the coupling ratio required and the degree of fusion achieved during the drawing stage. In the cross- section of the fused coupling region (Fig. 4), the surrounding optical fibers are situated symmetrically about the central fiber, in sufficiently close positions so that a desired output power uniformity occurs; the coupling ratio is determined at least in part by the diameter and length of the coupling region, as well as the degree to which the fibers have fused together. The degree of fusion can be observed by noticing the void regions 26 (Fig. 4), between the central fiber and any two neighboring surrounding fibers. Couplers of the invention can be made to have very little fusion of the optical materials, in which case the void regions are more pronounced and the fibers appear to retain more of their original boundaries. As the degree of fusion is increased the void regions become smaller and the boundaries of the individual fibers are reduced. Even though void regions may be present in any embodiment of the invention, the optical materials are considered to be fused into an essentially solid mass, which is referred to herein as a "unitary optical structure". Following the fuse-drawing process, the fibers that extend from the coupling region are bonded to a rigid, cylindrical substrate with an adhesive. The coupling region is suspended above the substrate. The bonds are designed to isolate and relieve the strain on the coupling region from external forces. This structure is then enclosed in a tube and an adhesive sealant is used at the end of the tube to protect the coupler. The entire structure may be encapsulated in a protective stainless steel tube. The inventors have discovered that by slightly reducing the diameter of the surrounding fibers relative to the circles described above, the optical performance of the fiberoptic couplers is greatly improved. By accommodating process-related shifts in the geometric relationship of the constituent fibers to one another, fiberoptic couplers have been fabricated with greater than 90% probability of having an output power uniformity of l.l dB or better. Subtle, yet important, changes in the geometric relationship of the fibers to one another occur during the fabrication of the coupler. These process-related shifts change the positions of the surrounding fibers relative to the central fiber, and to one another. When the coupler is twisted as shown in Fig. 3, a slight increase in the diameter of the cross-section of the surrounding fibers occurs, relative to the untwisted coupler of Fig. 1. This is a geometrical feature arising from the fact that an axially elongated cylinder which has a circular cross-section when cut perpendicular to its longitudinal axis will have an elliptical cross- section when cut along another axis. The diameter of the major axis of the ellipse is a minimum for the circular cross-section and increases with the size of the angle formed by the intersection of the circular cut axis (i.e., the axis transverse to the longitudinal axis of the cylinder representing the outer surface of a fiber) of a surrounding fiber and the circular cut axis of the central fiber. Other effects may also contribute to a mismatch in the sum of the diameters of the surrounding fibers with respect to the central fiber. The axial draw tension in conjunction with the helical twist of the surrounding fibers gives rise to radial forces that drive the surrounding fibers radially toward the central fiber. These forces cause the surrounding fibers to sink into the central fiber, which effectively reduces the diameter of the central fiber. If the reduction in the diameter of the central fiber occurs more rapidly than the deformation of a surrounding fiber with respect to its neighboring surrounding fibers, then there may not be sufficient space to close-pack all of the surrounding fibers around the central fiber. Diameter variations intrinsic to the constituent fibers (e.g., defects) may also result in insufficient space for close-packing. When at least one of the aforementioned mechanisms inhibiting close-packing is present, typically one or more of the surrounding fibers will not contact the central fiber, as shown in Fig. 5. In twisted couplers, the ellipticity of the surrounding fibers is too small to be observed directly, and indeed, the presence of the separation gap 28 is the clearest means of observing the effect. It has been observed that, most typically, all but one of the surrounding fibers will be mutually contacting, leaving one of the fibers with a separation gap 28. As shown in Fig. 5A, the diameters of one of the surrounding fibers may be reduced to permit close-packing of the constituent fibers. Alternatively, as shown in Fig. 5C, the diameters of each of the surrounding fibers may be reduced by different amounts to permit close- packing of the constituent fibers. The diameters of one or all of the surrounding fibers may be appropriately reduced, prior to fusion, to compensate for process-related shifts in the geometric relationship of the constituent fibers to one another. The sum of the diameters of the surrounding fibers should be toleranced to within about a micron for optimal coupler performance. When every fiber is reduced by the same amount, for a typical 1X6 fused tapered coupler, formed from fibers with diameters of 125 μm, the reduction in the diameter of the surrounding fibers relative to the sum of diameters of a ring of six circles that are closely- packed about the central fiber, in a plane transverse to the longitudinal axis of the central fiber, is of the order of about 0.5 μm. For N fibers positioned around one central fiber, the apparent increase in diameter Δd of each of the surrounding fibers, caused by the helical twist, can be expressed in general form by equation 2, below Δd = f(N,D,P) (2) in which D is the diameter of the central fiber, P is the pitch of the helical twist, and f is a generalized function of these parameters. Using simple geometric arguments, and ignoring secondary considerations, such as Poisson's ratio, the apparent increase in fiber diameter can be approximated by equation 3, below Δd = π 2 /(2P 2 ) (D/(l-sin(π/N))) 3 sin(τr/N) (3) In a preferred embodiment, each of the surrounding fibers is reduced in diameter by substantially Δd. Fig. 5C shows a coupler, before the surrounding fibers have been twisted about the central fiber, in which the diameters of each of the surrounding fibers have been reduced by Δd, resulting in a slight space (shown exaggerated) in the ring of surrounding fibers. As shown in Fig. 5D, close-packing of the constituent fibers is achieved in the coupler of Fig. 5C when the surrounding fibers have been helically twisted about the central fiber. Fig. 6 is a table that illustrates the change in diameter of each of the N surrounding fibers relative to the diameter of a circle in a ring of N identical circles closely-packed about the central fiber in a plane transverse to the longitudinal axis of the central fiber. The change in fiber diameter of each of the surrounding fibers is provided as a function of pitch P, that is required, in a preferred embodiment, to achieve the nine and seven fiber couplers shown in Figs. 2 and 5D, respectively. The original diameter of each of the surrounding fibers is assumed to be 125 μm. In the case where the central fiber is encircled by two rings of surrounding fibers, the reduction in diameter of each of the fibers in the inner ring of surrounding fibers can be approximated by equation 2. An approximate value for the reduction in diameter of each of the outer surrounding fibers (relative to an appropriately constructed corresponding ring of circles) can be similarly approximated by equation 2, as long as the fact that the effective diameter of the central fiber relative to the outer surrounding fibers is larger than the effective diameter of the central fiber relative to the inner ring of surrounding fibers is taken into account. The invention applies to all unitary couplers in which there is a central fiber encircled by one or more layers of surrounding fibers. The fibers in each layer can be sized to leave enough spacing to allow close- packing of the fibers. Referring to Fig. 7, a 1X3 coupler 37 is formed from six fibers surrounding a central optical fiber 38. Spacer fibers 40, 42, and 44 are fabricated from optically transparent material with a refractive index matched to the refractive index of the cladding of the neighboring optical fibers 46, 48, 50, including the central fiber 38. Since the spacer fibers do not have a core, optical power launched in the input fiber will not substantially couple to a spacer fiber, therefore optical power is not substantially lost through the spacer fibers. Different six-fiber coupler embodiments, formed from spacer fibers and optical fibers, are shown in cross-sectional views in Figs. 7A through 7G. The coupler arrangement of Fig. 7A may be used as a 1 x 6 or 1 x 5 coupler and, similarly, the coupler arrangement of Fig. 7B may be used as a l x 5 or l x 4 coupler depending upon the desired application, the desired coupling ratio, and whether the central input fiber is also utilized as an output fiber. In many applications in which light of several wavelengths (e.g., 1300 nm and 1550 run) is multiplexed for simultaneous transmission and distribution, it is important to fabricate couplers that have a substantially wavelength-independent coupling ratio. To obtain a substantially wavelength-independent coupling ratio, the coupler is drawn in the fusion process until the amount of input light distributed to the surrounding optical fibers is maximized. Alternatively, light of different wavelengths or modulation frequencies, may be carried by the constituent input optical fibers and coupled to a single output optical fiber. Fig. 7B shows an important 1 x 4 embodiment of the present invention. The central input, optical fiber is symmetrically arranged with respect to the output fibers so that each of the output fibers carries substantially equal amount of light. The symmetry of the optical fibers, their locations with respect to each other and the extent of the fuse-drawing govern the coupling ratio. The spacer fibers, which in general can have different sizes and refraction indexes, enable proper positioning of the optical fibers and easy manipulation of the fiber bundle. By achieving desired symmetrical or asymmetrical locations of the fiberoptic cores in the coupling region, standardized couplers with desired coupling ratios are fabricated. For example, for a fiberoptic coupler that exhibits a wavelength dependent coupling ratio, it is advantageous to form the coupling region with linearly arranged optical fibers, as shown in Figs. 7D or 7F. The spacer fibers maintain the desired distance of the fiber cores and thus the coupling ratio. Furthermore, the spacer fibers complete the close packed structure that is easy to handle during the coupler fabrication. Referring to Fig. 7G, in another embodiment the multi-fiber coupler is a 6 x 6 symmetrical coupler formed by six optical fibers that surround a central spacer fiber. The equal-diameter spacer fiber enables a easy positioning and symmetric packing of the optical fibers. A coupler with a different number of similarly packed N x N fibers is fabricated using a central spacer fiber of a different diameter. Referring to Fig. 8, in another embodiment, a separate protective ring of twelve spacer fibers surrounds a 1X4 coupler with twelve spacer fibers prior to the fusion process. During the fuse-drawing process, the spacer fibers form a glass barrier, i.e., an outer cladding, that protects the coupler and reduces the effects of the external environment on the coupler performance. As shown in Fig. 9, to achieve a substantially symmetrical arrangement and the appropriate close-packing of the optical fibers in a 1x8 coupler 52, six spacer fibers 54 surround a centrally located optical input fiber 56. Optical output fibers 58 are located in a second ring around the input fiber. The assembly of fibers is fused and drawn to form a tapered coupling region, as described above. Referring to Fig. 10, a nineteen fiber close- packed configuration is used to fabricate a 8 x 8 coupler. In the 8 x 8 coupler, the central optical fiber is a spacer fiber, and the optical fibers of the outer surrounding layer are used as both input and output fibers. Different nineteen-fiber coupler embodiments are shown in Figs. 10A through 10D. The arrangement of spacer fibers and optical fibers is selected depending upon the desired properties of the coupler. For example, couplers that include a larger number of optical fibers (e.g., 9 x 9, 10 x 10, etc.) are within the scope of the present invention. In addition to slightly reducing the diameters of one or more of the surrounding fibers, the diameter of the central fiber may be enlarged by e.g., adding a sleeve 60 of cladding material around the central optical fiber, as shown in Fig. 11. Four spacer fibers 62 and four optical fibers 64 form a close-packed ring of fibers around sleeve 60. Sleeve 60 is preferably made of material that has a refractive index that substantially matches the refractive index of the cladding of the optical fibers. Sleeve 60 may instead be made from material that has a refractive index that is greater than the refractive index of the cladding of the optical fibers. Referring to Figs. 13 through 13B, in another embodiment optical fibers and spacer fibers are arranged linearly side-by-side to form a ribbon-like structure. The multi-fiber coupler is again formed by fuse-drawing, as described above. The ribbon-like structure may be twisted in the fusion process to provide improved coupling between the fibers. Since the coupling ratio at different wavelengths depends on the distance between the fiber cores, a desired wavelength dependent coupling ratio can be achieved by selecting a spacer fiber with a proper diameter or a suitable number of spacer fibers separating the optical fibers. This arrangement may be used to fabricate a wavelength-dependent multiplexer (WDM) . Referring to Figs. 12 and 12A, the use of spacer fibers enables fabrication of several fiberoptic couplers integrated into one rigid structure. The individual couplers are separated by one or more spacer fibers depending upon the required separation. The individual couplers are optically "insulated" from each other by the volume of optical material provided by the spacer fibers. The structure is fuse-drawn in a single location to form an integrated rigid structure. This embodiment allows close-packing of several optical couplers, each having a selected geometry and a selected coupling ratio. It is within the scope of this invention to use spacer fibers of different sizes and shapes made of substantially homogeneous optical material. A hollow- cylindrical fiber may be also used as the spacer that maintains a desired distance between the fibers. In a fiberoptic structure that includes several fiberoptic couplers (Figs. 12 through 12A) , the hollow cylindrical spacer fibers also serve to provide an air cladding for the coupling region. Other embodiments are within the scope of the claims. For example, the coupling region of the above- described couplers can be formed by a bonding process in which bonding is achieved by using a 5-minute epoxy, UV epoxy or other suitable adhesives. Before, or after, the bonding material is applied, the fiber bundle can be tied together to exert an external radial force on the fibers. It is within the scope of the present invention to use etched or un-etched optical fibers.

Claims: CLAIMS: - 2 3 - 1. A fused fiberoptic coupler formed from axially elongated, cylindrical fibers of transparent substance each having a circular transverse cross-section of preselected diameter, the fibers including a central fiber and at least one ring of N surrounding fibers, at least some of said fibers being optical fibers having a core and a surrounding cladding, said coupler being formed by the processes of heating and drawing said fibers, the sum of the diameters of said surrounding fibers being slightly less than the sum of diameters of a ring of N circles closely-packed about said central fiber in a plane transverse to the longitudinal axis of said central fiber in a manner providing, during manufacture, slight space in said ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture, a close-packed relationship of the fibers to one another is obtained. 2. The coupler of claim 1 being further characterized in that a fusing process employed in the manufacture of the coupler uses helical twist of the surrounding fibers to obtain stable contact between all neighboring fibers prior to fusion so that during the fusing process said surrounding fibers fuse together and with the central fiber into a unitary optical structure, and the amount by which said sum of diameters of said surrounding fibers is less than said sum of diameters of said ring of N circles is determined, at least in major part, to accommodate the slight elliptical shape of the cross-section of the surrounding fibers, taken transversely to the axis of the central fiber, attributable to their helical twist relative to the central fiber, so that said close-packed relationship of the fibers to one another is obtained. 3. The coupler of claim 2 wherein said helical twist has a pitch in the range of about 0.5 cm to 5 cm and said amount by which said sum of diameters of said surrounding fibers is less than said sum of diameters of a ring of N circles is substantially inversely proportional to the square of said pitch. 4. The coupler of claim l wherein each of said N circles has a diameter d in said fused region, said central fiber has a diameter D in said fused region, and said close-packing of said N circles conforms substantially to a ratio of said diameters equal to the value d/D = sin(τ/N)/ ( l-sin (7t/N) ) . 5. The coupler of claim 1 wherein, after manufacture, each of said surrounding fibers has substantially the same diameter. 6. The coupler of claim 1 wherein, after manufacture, at least two of said surrounding fibers have different diameters. 7. The coupler of claim l wherein said processes of heating and drawing said fibers are performed in a fused region of said coupler, said central fiber and said surrounding fibers are formed of identical fibers that extend beyond said fused region, the difference in diameters of said fibers in said fused region being the result of a substantially uniform increase in the diameter of said central fiber or a substantially uniform decrease in the diameter of said surrounding fibers prior to fusion. 8. The coupler of claim 1 wherein said processes of heating and drawing said fibers are performed in a fused region, said central fiber and said surrounding fibers being formed of identical fibers extending beyond the fused region, at least one of said surrounding fibers has a smaller diameter than the diameter of said central fiber in said fused region, the difference in diameters in said fused region being the result of a substantially uniform reduction in the diameters of said at least one surrounding fiber or a substantially uniform increase in diameter of said central fiber prior to fusion. 9. The coupler of claim 7 wherein said uniform decrease in fiber diameter is achieved by etching, drawing or a combination of etching and drawing. 10. The coupler of claim 7 wherein said central fiber comprises an optical core surrounded by a cladding having a refractive index, and said increase in said central fiber diameter is achieved, prior to fusion, by the addition of a material to the outer surface of said central fiber, said material having a refractive index greater than or about equal to the refractive index of said cladding. 11. The coupler of claim 1 wherein said surrounding fibers are preselected to have a smaller diameter than the diameter of said central fiber. 12. The coupler of claim 1 wherein said central fiber is preselected to have a larger diameter than the diameters of said surrounding fibers. 13. The coupler of claim 1 having a larger bandwidth of optical wavelength response relative to a coupler formed of the same fibers without said accommodation of said process-related shifts. 14. The coupler of claim 1 having an improved output power uniformity relative to a coupler formed of the same fibers without said accommodation of said process-related shifts. 15. The coupler of claim l wherein said process- related shifts are based at least in part upon an effective reduction in diameter of said central fiber caused by inward indenting pressure of the surrounding fibers upon said central fiber during the fusing step of manufacture. 16. The coupler of claim 1 wherein said central fiber is constructed to serve as an input port for optical power, and said surrounding fibers are each constructed to function as an output port, said coupler having a predetermined desired coupling ratio. 17. The coupler of claim 16 wherein said desired coupling ratio is characterized in that the fraction of output power coupling to each of said surrounding fibers is substantially 1/N. 18. The coupler of claim 17 wherein said desired coupling ratio is characterized in that the fraction of output power that couples to each of said surrounding fibers is substantially 1/(N+1). 19. The coupler of claim 1 wherein at least one of said fibers comprises an optical core surrounded by a cladding having a cladding refractive index, and one of said fibers being adjacent to said at least one fiber is a spacer fiber having a refractive index substantially matched to said cladding refractive index. 20. The coupler of claim 1 wherein at least said central fiber is a spacer fiber and at least two of said surrounding fibers are optical fibers. 21. The coupler of claim 1 wherein at least said central fiber is an optical fiber and at least one of said surrounding fibers is a spacer fiber. 22. A 1XM (M taking a value of 2, 3, 4, 5 or 6) fiberoptic coupler comprising: M+l substantially equal diameter optical fibers each having a core and a surrounding cladding, and 6-M spacer fibers each of which having a refractive index substantially matched to the refractive index of the cladding of said optical fibers, one of said M+l optical fibers being located in the center and the other M optical fibers with said 6-M spacer fibers forming a ring of six fibers around said central fiber in a close-packed configuration characterized in that the sum of the diameters of said surrounding fibers being slightly less than the sum of diameters of a ring of N circles closely-packed about said central fiber in a plane transverse to the longitudinal axis of said central fiber in a manner providing, during manufacture, slight space in said ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture, a close-packed relationship of the fibers to one another is obtained. 23. A 1XN (N taking an integer value greater than 6) fiberoptic coupler comprising: N substantially equal diameter optical fibers each having a core and a surrounding cladding, and one central optical fiber. said N optical fibers forming a ring of fibers surrounding said central fiber in a close-packed configuration characterized in that the sum of the diameters of said surrounding fibers being slightly less than the sum of diameters of a ring of N circles closely- packed about said central fiber in a plane transverse to the longitudinal axis of said central fiber in a manner providing, during manufacture, slight space in said ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture, a close-packed relationship of the fibers to one another is obtained. 24. A fiberoptic coupler comprising: a multiplicity of optical fibers each having a core and a surrounding cladding, and a multiplicity of spacer fibers each having a refractive index substantially matched to the refractive index of the cladding of said optical fibers, one of said optical fibers being located in the center surrounded by a first ring of six optical and spacer fibers, and a second ring of twelve optical and spacer fibers located substantially symmetrically around said central optical fiber and said first ring forming a close-packed configuration characterized in that the sum of diameters of a ring of N circles closely-packed about said central fiber in a plane transverse to the longitudinal axis of said central fiber in a manner providing, during manufacture, slight space in said ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture, a close-packed relationship of the fibers to one another is obtained. 25. A fiberoptic coupler comprising a central spacer fiber that has no optical core, multiple surrounding fibers at least two of which being optical fibers that have optical cores and surrounding cladding, said multiple fibers being arranged, over a region extending axially through a limited length coupling region, in a close-packed ring of fibers that surrounds said central spacer fiber wherein said multiple and central fibers contact neighboring fibers, and said coupling region comprising a fused region in which all of said surrounding fibers and said central fiber, in their respective regions of contact, are fused together forming a unitary structure adapted to couple propagating modes of light between said optical fibers. 26. The fiberoptic coupler of claim 25 wherein said central spacer fiber has a refractive index substantially matched to the refractive index of the cladding of at least one of said at least two optical fibers. 27. The fiberoptic coupler of claim 25 wherein the distance separating the constituent optical fibers is selected to achieve wavelength-dependent coupling of modes between the constituent optical fibers. 28. The fiberoptic coupler of claim 25 wherein said multiple surrounding fibers are of substantially equal diameter. 29. The fiberoptic coupler of claim 28 wherein said central fiber has a diameter substantially equal to the diameter of said multiple surrounding fibers. - SO - SO. The fiberoptic coupler of claim 28 wherein six of said multiple surrounding fibers form a close- packed ring around said central spacer fiber. 31. The fiberoptic coupler of claim 25 wherein said close-packed ring of fibers is surrounded by a second ring of close-packed spacer fibers. 32. The fiberoptic coupler of claim 25 wherein said coupling region is formed by a fuse-drawing process. 33. The fiberoptic coupler of claim 25 wherein the length of said coupling region is selected to achieve a desired coupling ratio between the constituent optical fibers. 34. The fiberoptic coupler of claim 25 wherein at least one constituent spacer fiber comprises a solid cylindrical fiber made of substantially homogeneous optical material. 35. The fiberoptic coupler of claim 25 wherein the constituent optical fibers of said coupler are arranged to form at least two substantially optically segregated couplers each comprising at least two optical fibers, said at least two substantially segregated couplers being separated by one or more spacer fibers. 36. A 1x4 fiberoptic coupler comprising: five substantially equal diameter optical fibers each having an optical core and a surrounding cladding; two spacer fibers, each having no optical core; one of said optical fibers being a central fiber and the other four of said five optical fibers and said two spacer fibers forming a close-packed ring of fibers surrounding said central fiber in a limited length coupling region, wherein the constituent fibers in the six fiber ring are arranged in the following fiber sequence: a group of two adjacent optical fibers, a spacer fiber, a second group of two adjacent optical fibers, and a second spacer fiber; wherein the constituent fibers of said ring contact neighboring fibers and said central fiber, and said coupling region is created by a fuse-drawing process in which the fibers of said ring and said central fiber are fused together in their respective regions of contact forming a unitary structure adapted to couple propagating modes of light between said optical fibers. 37. The 1X4 fiberoptic coupler of claim 36 wherein each of said two spacer fibers has a refractive index substantially matched to the refractive index of the cladding of each of said five optical fibers. 38. A 1x8 symmetric fiberoptic coupler comprising: nine equal diameter optical fibers each having an optical core and a surrounding cladding, ten spacer fibers, each having no optical core, and each having a refractive index substantially matched to the refractive index of the cladding of said optical fibers, one of said optical fibers located in the center surrounded by an inner, close-packed ring of six of said ten spacer fibers and a second ring of twelve fibers formed by the remaining eight of said nine optical fibers and the remaining two of said ten spacer fibers located substantially symmetrically around said central optical fiber forming a close-packed arrangement, and said fibers extending axially through a limited length coupling region, created by a fuse-drawing process in which the constituent optical and spacer fibers are fused together in their respective regions of contact forming a unitary structure adapted to couple propagating modes of light between said optical fibers. 39. A fiberoptic coupler assembly comprising: multiple optical fibers each having an optical core and a surrounding cladding, multiple spacer fibers, each having no optical core, and each having a refractive index substantially matched to the refractive index of the cladding of at least one of said multiple optical fibers, said optical fibers arranged in substantially optically segregated groups of two or more adjacent optical fibers, said groups being separated by at least one spacer fiber, wherein the constituent fibers of said coupler are arranged side-by-side and extend axially through a limited length coupling region, and said coupling region being created by laterally fusing together the constituent optical and spacer fibers in their respective regions of contact forming a unitary coupler adapted to couple propagating modes of light between the optical fibers within each of said groups, said spacer fibers being adapted to substantially eliminate coupling of modes of light between the different groups of fibers. 40. The fiberoptic coupler of claim 25, 26, 27, 28, 29, 36, 37, 38 or 39 wherein said optical fibers are single-mode optical fibers. 41. The fiberoptic coupler of claim 25, 26, 27, 28, 29, 36, 37, 38, or 39 wherein said optical fibers are multi-mode optical fibers. 42. A method of fabricating a fiberoptic coupler comprising the steps of: (a) providing a multiple surrounding fibers at least two of which being optical fibers that have optical cores and surrounding cladding, (b) providing a central spacer fiber, having no optical core, (c) arranging said multiple surrounding fibers over a region extending axially through a limited length coupling region, in a close-packed ring of fibers that surrounds said central spacer fiber wherein said multiple surrounding and central fiber contact neighboring fibers, and (d) fusing together said multiple surrounding and central fibers in their respective regions of contact, in said limited length coupling region, to form a unitary structure having a desired coupling ratio between the constituent optical fibers. 43. The method of claim 42 wherein said central fiber that is provided has a refractive index substantially matched to the refractive index of the cladding of at least one of said at least two optical fibers. 44. The method of claim 42 wherein said step of arranging said multiple surrounding fibers comprises arranging the constituent fibers to achieve a desired distance separating the constituent optical fibers in a manner to achieve wavelength-dependent coupling of modes between the constituent optical fibers. 45. The method of claim 42 wherein said multiple surrounding fibers that are provided are of substantially equal diameter. 46. The method of claim 45 wherein said central fiber that is provided has a diameter substantially equal to the diameter of said multiple surrounding fibers. 47. The method of claim 42 wherein said step of arranging comprises arranging six of said multiple surrounding fibers around said central spacer fiber in a close-packed ring. 48. The method of claim 42 wherein the step of arranging said multiple surrounding fibers comprises arranging six of said multiple surrounding fibers in an inner close-packed ring of fibers wherein each of said fibers in said inner ring contacts neighboring surrounding fibers and said central fiber, and arranging a second close-packed ring of fibers around said inner ring of fibers. 49. The method of claim 42 wherein the step of arranging said multiple surrounding fibers comprises arranging nineteen substantially equal diameter fibers so that said central spacer fiber is surrounded by a first ring of six close-packed fibers that is in turn surrounded by a second ring of twelve close-packed fibers. 50. The method of claim 49 wherein at least one of the eighteen surrounding fibers that are provided is a spacer fiber that has no optical core. 51. The method of claim 42 wherein said central fiber that is provided is a spacer fiber that has no optical core. 52. The method of claim 42 wherein the step of fusing is performed by fuse-drawing to form a tapered laterally fused coupling region. 53. The method of claim 42 wherein said optical fibers are single-mode optical fibers. 54. The method of claim 42 wherein said optical fibers are multi-mode optical fibers. 55. A fiberoptic coupler comprising a central fiber, and multiple surrounding fibers at least two of which being optical fibers that have optical cores and surrounding cladding, at least one of said central and multiple surrounding fibers being a spacer fiber that has no optical core, said multiple fibers being arranged, over a region extending axially through a limited length coupling region, in a close-packed ring of fibers that surrounds said central fiber wherein said central and multiple surrounding fiber contact neighboring fibers, said multiple surrounding fibers comprising an inner ring of fibers that surrounds said central fiber in a close-packed configuration wherein each of said fibers in said inner ring contacts neighboring surrounding fibers and said central fiber, and a second ring of fibers surrounding said inner ring of fibers, said coupling region comprising a fused region in which said multiple surrounding fibers and said central fiber, in their respective regions of contact, are fused together forming a unitary structure adapted to couple propagating modes of light between said optical fibers. 56. The fiberoptic coupler of claim 55 wherein said second ring comprises a close-packed ring of spacer fibers. 57. A fiberoptic coupler comprising an inner axially elongated spacer fiber having no optical core, and first and second axially elongated optical fibers each having an optical core and a surrounding cladding, said optical and spacer fibers being arranged linearly side-by-side with said spacer fiber separating and contacting said first and second optical fibers in a limited length coupling region, said coupling region comprising a fused region in which said optical and spacer fibers, in their respective regions of contact, are fused together forming a unitary ribbon-like structure, wherein the transverse distance separating the optical cores of the optical fibers is selected to achieve a desired wavelength-dependent coupling ratio between the constituent optical fibers of said coupler. 58. The method of claim 42 wherein the diameter of said central spacer fiber is selected to enable close- packing of said multiple surrounding fibers. 59. A method of fabricating a fiberoptic coupler comprising the steps of: (a) providing a central fiber; (b) providing multiple surrounding fibers at least two of which being optical fibers each having an optical core and a surrounding cladding, at least one of said central and multiple surrounding fibers being a spacer fiber without an optical core; (c) arranging said multiple fibers, over a region extending axially through a limited length coupling region, in a close-packed ring of fibers surrounding said central fiber wherein said central and multiple surrounding fiber contact neighboring fibers, wherein arranging said ring of multiple surrounding fibers comprises surrounding said central fiber with an inner ring of said multiple surrounding fibers in a close-packed configuration wherein each of said fibers in said inner ring contacts neighboring surrounding fibers and said central fiber, and surrounding said inner ring of fibers with a second ring of fibers; and (d) fusing together, in said coupling region, said multiple surrounding and said central fibers in their respective regions of contact thereby forming a unitary structure adapted to couple propagating modes of light between the constituent optical fibers. 60. A method for forming a 1X4 fiberoptic coupler comprising the steps of: (a) providing five substantially equal diameter optical fibers each having an optical core and a surrounding cladding; (b) providing two spacer fibers each without an optical core and each having a diameter substantially equal to the diameter of said optical fibers; (c) arranging four of said five optical fibers and said two spacer fibers in a close-packed ring around a central fiber consisting of the remaining one of said five optical fibers in the following sequence: a group of two adjacent optical fibers, a spacer fiber, a second group of two adjacent optical fibers, and a second spacer fiber, wherein each of the fibers in said close-packed ring contact neighboring fibers in the ring and said central optical fiber; and (d) fusing together, in said coupling region, the fibers in said ring and said central fiber in their respective regions of contact thereby forming a unitary structure adapted to couple propagating modes of light between the constituent optical fibers. 61. The fiberoptic coupler of claim 60 wherein said multiple surrounding and central fibers comprise nineteen substantially equal diameter fibers with said central fiber surrounded by an inner close-packed ring of six of said multiple surrounding fibers and a second close-packed ring of twelve of said multiple surrounding fibers. 62. The fiberoptic coupler of claim 60 wherein said central fiber is an optical fiber and at least one of said multiple surrounding fibers is a spacer fiber having no optical core. 63. The fiberoptic coupler of claim 60 wherein said central fiber is a spacer fiber having no optical core. 64. The subject matter of claims 25, 36, 37, or 40, wherein the sum of the diameters of the fibers surrounding said central fiber are slightly less than the sum of the diameters of a number of circles, equal to the number of fibers surrounding said central fiber, arranged in a close-packed ring about said central fiber in a plane transverse the longitudinal axis of said central fiber in a manner providing, during manufacture, slight space in said ring of surrounding fibers to accommodate process-related shifts of the geometric relationship of the fibers to one another, so that, after manufacture, a close-packed relationship of the fibers to one another is obtained.

Inventors: CRYAN COLM V, MAACK DAVE R, STOWE DAVID W, CRYAN, COLM, V., MAACK, DAVE, R., STOWE, DAVID, W.
Applicants: PORTA SYSTEMS CORP, PORTA SYSTEMS CORPORATION
Application year: 1994
Main classification: G02B6
IPC codes: G02B6